1. Field of the Invention
The present invention relates to a digital multimeter for measuring current, voltage, etc., and particularly to an improvement in its water proof and dust proof enclosure.
2. Description of the Related Art
Digital multimeters are typically provided with elements such as a rotary switch, a display window, and other elements disposed on the front surface of an enclosure so that various kinds of electrical parameters can be measured by selecting a desired parameter and a proper range via the rotary switch. Inside a digital multimeter are electrical components such as a battery and circuit boards on which integrated circuits and other electrical elements are mounted. To protect them from external dust, moisture, etc., various types of water proof and dust proof techniques are known in the art.
One such conventional digital multimeter is disclosed in U.S. Pat. No. 4,951,834. As shown in FIG. 1 of the above patent, a digital multimeter which has an enclosure with a size that can be carried by one hand, wherein component elements such as a circuit board and a battery are disposed inside the enclosure. On the front surface of the enclosure, there are disposed operating elements such as a rotary switch and press keys used to select a parameter to be measured and its measuring range. Furthermore, a display panel for displaying the measured result is also disposed on the front surface of the enclosure.
The enclosure of the digital multimeter comprises upper and lower enclosures. The sealing at the interface between the connection faces of the upper and lower enclosures is achieved by inserting a gasket or packing between the connection faces and pressing it from both sides. Since the gasket or the packing is placed and pressed between the upper and lower enclosures, the upper and lower enclosures are strongly fastened to each other via a plurality of fastening screws so that they can withstand the opposing force of the gasket. The sealing between the rotary switch and the upper enclosure is also achieved by inserting an elastic sealing material such as a gasket between them and pressing it from both sides. Furthermore, a display window with a transparent glass plate is disposed in the upper enclosure wherein the boundary between the periphery of the transparent glass plate and the upper enclosure is sealed by means of ultrasonic welding.
The above-described conventional sealing mechanism has disadvantages as will be described below. Regarding the sealing mechanism between the upper and lower enclosures, since an elastic sealing material such as a gasket is placed and pressed between these upper and lower enclosures, it is required that the upper and lower enclosures be strongly connected to each other using a plurality of screws so as to ensure that the upper and lower enclosures are not separated by the opposing force of the elastic sealing material. However, this produces a problem that if the number of fasting screws is reduced to reduce production cost, the sealing quality is degraded.
Another problem regarding the sealing mechanism of the rotary switch is that the rotary switch is often fixed to the upper enclosure at some angle with respect to its surface due to inaccuracies in production of the rotary switch or due to positioning error of the rotary switch, which results in an imperfect seal.
Furthermore, the peripheral boundary of the transparent glass plate attached to the surface of the enclosure is not sealed perfectly, thus, water or dust may come into the inside of the enclosure through the boundary.
In view of the above, it is an object of the invention to provide an excellent sealing mechanism for sealing the connection interface between the upper and lower enclosures forming the enclosure of a digital multimeter.
It is another object of the invention to provide a technique for forming a more reliable seal between the enclosure and operating elements such as a rotary switch attached to the enclosure of a digital multimeter.
It is still another object of the invention to provide a technique for forming a more reliable seal between the enclosure and a display window formed in the enclosure of a digital multimeter.